I've created a small patch to add karma feature into current FUDforum 3.0.3. Initially I thought it should be a plugin, but it turned out to be easier to write a patch.
The patch require modification of the database (attached SQL script) and some new messages (also in the patch)

Features:

* full AJAX support (no page reloads)
* use current Rate Topic and Can rate permissions
* users with "can rate" permission can add +1 or -1 to the author of the post
* author can't change karma for own posts
* forum admin can remove karma rating (click on Karma counter next to the user)